There's a fine art to sweeping the pleats as you sit (or re-sweeping after you've sat, by lifting yer bum up again). It's critical to maintaining your kilt in an orderly and tidy fashion. Plus it makes you seem like a kilted pro.

Funny story, I actually got a compliment from an older fellow at a Highland Games when I went to sit down in a chair at one of the clan tents to chat for a while. He made a point to tell me that he noticed I sat 'correctly' by sweeping my pleats and punching down the apron so I didn't flash everybody. Made me feel pretty good!

It takes a little bit of practice, but not much. It just becomes a fluid motion when you sit down. Sweep the pleats with one hand, and use the other to press the apron down. And once you're seated, especially if you've been sitting for a while, you can simply lift up and re-sweep to make sure everything's smooth under there. It becomes second-nature after a while.